[en] XY-like spin-glass models without time-reversal symmetry are considered, with attention focused on the gauge glass and the Dzyaloshinsky-Moriya XY spin glass. A mean-field approximation of the replicated Ginzburg-Landau free energy of these models is used to show that, due to the lack of time-reversal symmetry, these models have two soft (massless) modes at the transition. A consequence of this result is that all XY-like spin glasses lacking time-reversal symmetry belong to the same universality class, but which is a different one from that found for Ising and isotropic vector spin glasses

[en] A model of disordered superconducting grains in a strong magnetic field, a related random-gauge glass model, and two versions of an XY spin-glass model with random spin-orbit interactions are discussed. Results from an extensive zero-temperature finite-size scaling study of the defect-wall energy are presented to show that these four models probably belong to a new spin-glass universality class and that the lower critical dimension for a vortex-glass transition at nonzero temperature in these models is slightly below three dimensions

[en] A zero-temperature Migdal-Kadanoff renormalization-group scheme is used to investigate two classical spin-glass lattice models of a random superconductor in a magnetic field. We show that the two models considered here become the same under renormalization and belong to a different universality class from the Ising and XY spin glasses. We find that, in contrast with results from recent Monte Carlo simulations, the lower critical dimension for a nonzero-temperature vortex-glass transition in a random superconductor is greater than 3

[en] Discovered in 1997, spin ices are magnetic materials that display a low temperature state characterized by a nonzero residual magnetic entropy that is intimately related to the proton disorder entropy of common water ice - hence the name spin ice. In this article, we review the salient aspects of these fascinating systems that have sustained the interest of theorists and experimentalists alike for nearly fifteen years. (author)

[en] In R₂Ti₂O₇ compounds, rare earth R ions with well localized 4f moments occupy a pyrochlore lattice with corner sharing tetrahedra. This lattice is highly frustrated for antiferromagnetic interactions, and for ferromagnetic interactions between ions with a strong local Ising anisotropy. The high frustration leads to exotic magnetic states, such as spin liquid or spin ice states, whose stability is governed by a delicate balance between exchange, anisotropy and dipolar interactions. At ambient pressure, Ho₂Ti₂O₇ behaves as a 'dipolar spin ice', with a magnetic state mapping that of the ice lattice. In contrast, Tb₂Ti₂O₇ behaves as a 'spin liquid', retaining short range liquid-like spin correlations down to at least 70 mK, despite a Curie Weiss temperature of ∼19 K. The energy balance is modified by pressure, since magnetic interactions depend on interatomic distances in different ways. We have studied the influence of an applied pressure for the first time, up to 9 GPa, in the temperature range 1.4 K < T < 300 K, by high pressure neutron diffraction. In Ho₂Ti₂O₇, the spin ice state remains stable up to at least 5 GPa. In Tb₂Ti₂O₇, the spin liquid state also remains stable up to 5 GPa, but under higher pressures novel features appear in the neutron spectra, pointing out drastic changes in the microscopic spin arrangement. From our first results, high pressure shows as a new and essential tool to investigate the magnetic interactions responsible for the novel magnetic states observed in highly frustrated magnets. This study was performed using a technique recently developed in Laboratoire Leon Brillouin, which allows one to determine magnetic order up to much higher pressures than in conventional neutron measurements. (author)

[en] In the amorphous ( Fe_1-x Mn_x)₇₅ P₁₆ B₆ Al₃ alloys, muon and neutron depolarization data, combined with the results of small angle neutron scattering, magnetization and Moessbauer spectroscopy, probe the existence of three distinct magnetic transitions at T_C, T_K and T_F (T_F < _K < _C)

[en] In a system where magnetic ions occupy the vertices of edge or corner sharing triangular units, the natural antiferromagnetic coupling between ions is geometrically frustrated. A wide variety of interesting magnetic behaviour has been observed in pyrochlores, where magnetic ions form a network of corner sharing tetrahedra. The low temperature spin dynamics of a number of pyrochlores A₂B₂O₇ have been investigated using the technique of μ SR. For example, Y₂Mo₂O₇ shows a transition to a disordered magnetic state similar to a spin glass at T_F=22 K. However, unlike conventional metallic spin glasses, a non-zero muon spin depolarization rate is observed to persist well below 0.1 T_F. These results suggest that there is a finite density of states for magnetic excitations in this system near zero energy